p-Toluic acid is an important chemical product that is widely used as a raw material for producing sensitizing dyes, fluorescent dyes, anticorrosives, pigments, agricultural chemicals, and the like. The purity of standard-quality p-toluic acid is said to be 98%, and it is a chemical product generally required to have high purity. In particular, especially high purity is required for p-toluic acid for use in pharmaceuticals.
Industrial production of p-toluic acid is carried out by air oxidation of p-xylene in the presence of a cobalt salt, manganese salt, or cerium salt catalyst at 150° C. to 200° C. followed by separation and purification of p-toluic acid from the resulting crude reaction liquid (Patent Document 1).
Patent Document 2 discloses a method for producing p-toluic acid by a simple separation operation. In this method, air oxidation of p-xylene is carried out in the presence of water, and the resulting product is recovered by filtration after cooling, followed by washing with toluene. By this, a solid mainly containing p-toluic acid is obtained.
Several methods for producing highly pure p-toluic acid have been disclosed. For example, a production method in which p-xylene is removed by distillation from a p-xylene oxidation reaction crude liquid, and the resulting product is subjected to hot sulfuric acid treatment at 200° C., esterification treatment at 200° C., and then steam distillation at 230° C. has been disclosed (Patent Document 3). A production method in which p-xylene is removed by distillation from a p-xylene oxidation reaction crude liquid, and the resulting product is subjected to base treatment at 138° C. and water extraction, followed by removing the resulting salt, cooling the resulting product to allow precipitation of p-toluic acid, and then washing the p-toluic acid with hexane has been disclosed (Patent Document 4). A production method in which a p-xylene oxidation reaction crude liquid is brought into contact with an aqueous base solution and water at 120° C., and the resulting organic phase is cooled to allow precipitation of p-toluic acid, followed by washing of the p-toluic acid with p-xylene has also been disclosed (Patent Document 5).
In recent years, because of problems such as possible depletion of fossil resources in the future and global warming due to emission of greenhouse gases, methods for producing chemical products using, as raw materials, substances inducible from biomass resources, which are regenerative resources, have been demanded. As such a method, Patent Document 6 discloses a microorganism having an ability to biosynthesize p-toluic acid from glyceraldehyde-3-phosphate and pyruvic acid, and a method using the microorganism. Glyceraldehyde-3-phosphate and pyruvic acid are compounds produced by fermentation of biomass resources.